The study of crystal growth with the electron microscope III. Growth-step patterns and the relationship of growth-step height to molecular structure in n -nonatriacontane and in stearic acid

Abstract

The exact relationship between molecular association in solution and growth-step height was obtained from measurements of shadowcast electron micrographs. Crystals of the paraffin n -nonatriacontane, C 39 H 80 , give a value of growth-step height of 45 ± 10 Ȧ, the unimolecular value. Similarly, crystals of stearic acid, C 17 H 35 COOH, give a value of 47 ± 10 Ȧ, the bimolecular value. There is thus strict correspondence between the unimolecular nature of n -nonatriacontane molecules in solution and unimolecular n -nonatriacontane growth-steps and between the bimolecular nature of stearic acid molecules in solution and bimolecular stearic acid growth-steps. In crystals of both compounds, grown from a dislocation of length any multiple of the length of the units in solution, multimolecular growth-steps were not encountered. Instead the growth pattern dissociated into a succession of unimolecular steps in n -nonatriacontane crystals and bimolecular steps in stearic acid crystals. There is thus strong evidence that the size of the unit in solution and not the length of the parent dislocation is the responsible factor in deciding the height of growth-steps. The crystals examined show a variety of growth-step patterns—spirals, layers, halfsteps and flat-tops. In layered crystals which show no sign of spiral growth there is strong internal evidence that the crystals have in fact grown by a spiral mechanism. A series of micrographs showing crystals in a state of transition makes it clear that the screw dislocation responsible for growth has been cancelled out by the formation of an equal and opposite dislocation. Numerous examples of this cancellation process have been encountered either at the end of all growth or as a relapse when one of the dislocations in a pair of the same sense has been cancelled during growth. This evidence for the growth and movement of dislocations in real crystals shows that crystals with molecularly flat surfaces may well be produced by a mechanism based on screw dislocations. The fact that no spiral growth-steps can be observed is no longer adequate proof that a spiral growth mechanism did not produce the crystal in question.